The experiments are designed to study the re-establishment of connections between two populations of uniquely identifiable cells which are originally connected in a very precise pattern. The superficial flexor muscle of the crayfish Procambarus clarkil is innervated by six axons which make connectons with 40-50 muscle fibers in a pattern that depends on the position of the individual muscle fibers on the surface of the muscle. I propose to study the regeneration of neuromuscular connections in this system. Crustacean neurons can regenerate by the fusion of the two cut ends of the axons and regrowth of the proximal stump, either in the presence of a surviving distal stump or in its absence. The first experiments are designed to obtain these three types of regeneration on the same muscle system, for each one will provide insight into different aspect of the regenerative process. The second series of experiments will analyze the specificity of the regenerated connectivity map. Specificity of the connections is defined in terms of three physiological measurements made on each muscle fiber for each one of the six axons: presence or absence of a connection, mean size of the junction potentials and the facilitation characteristics of the junctions when the axons are stimulated at different frequencies. These measurements will be made on some identified muscle fibers before the nerve is cut and after regeneration has taken place. The physiological measuremenn that remains more constant when comparing animals before and after the operations will be used for the last series of experiments. The position and orientation of the postsynaptic field will be changed; regeneration will take place in fibers which still have functional synapses. By comparing the physiological parameter selected when normal regeneration occurs with the same parameter when regeneration occurs under changed conditions, one would be able to study the effect of muscle fiber position or the existence of old terminals on the specificity of the regenerated process. These results will give an indication of the possible developmental mechanisms that were responsible for the formation of the original specific neuromuscular connections.